Split spaghetti squash in half. Scoop out the seeds. Cook however you normally would spaghetti squash. I like to place mine in a glass dish with a bit of water. Cover and microwave for about 15 minutes. While cooking, mix the black beans, corn, and onion in a bowl. Leave it sitting out. Once squash is tender, use a fork to get the squash strands out. Place squash strands in a saucepan, 1 cup of water, and taco seasoning. Simmer for 5 minutes. Meanwhile, place taco shells in the oven to crisp.
Fill tacos with seasoned spaghetti squash, black bean mixture, cheese, and spinach.